1. Field of the Invention
This invention relates generally to olefin polymerization and, more particularly, this invention relates to a method of polymerizing and copolymerizing ethylene at high temperatures.
2. Brief Description of the Prior Art
In recent years, transition metal-based catalyst systems have been developed which allow the production of high density polyethylene (HDPE) and so-called linear low density polyethylene (LLDPE) under high pressure, high temperature conditions which had previously been used for the production of low density polyethylene (LDPE). An important economic advantage which has resulted from these catalyst systems is the capability of using a single reaction vessel for the production of different polyethylene products.
However, prior transition metal-based ethylene polymerization and copolymerization systems, when operated at high temperatures, (i.e., above about 160.degree. C.) exhibit several disadvantages. Even though catalyst reactivity (based on transition metal content) tends to increase with a rise in temperature, catalyst life time tends to decrease with an increase in temperature. Operation at high temperatures generally results in low yields due to thermal instability of the catalyst, and product polymer melt index is generally relatively high, and thus the molecular weight low. Such resins are not generally strong. This problem is especially acute in the copolymerization of ethylene with 1-olefins.
Further, many prior high temperature catalyst systems utilize alkyl aluminum chloride or similar halogen-containing cocatalysts, thus resulting in a high halogen content in the catalyst system which in turn contributes to high halide contents (in the form of catalyst residues) in polymer products. Such halide-containing residues are highly corrosive.
One method of high temperature (e.g., 175.degree.-300.degree. C.) ethylene polymerization is described in U.S. Pat. No. 2,882,264 (Apr. 14, 1959) to Barnes et al. In the Barnes et al patent, a simple Ziegler catalyst is used at a pressure greater than about 500 atmospheres. The catalyst is broadly described as the reaction product of a transition metal compound of Group IVB, VB or VIB of the Periodic Table and an organometallic cocatalyst. A typical cocatalyst is triisobutylaluminum (TIBAL). The best reactivity of the Barnes et al system corresponds to 2120 g/gTi/minute, which is very low.
U.S. Pat. No. 4,210,734 (July 1, 1980) to Machon discloses several magnesium-titanium combinations as being useful in high temperature, high pressure ethylene polymerization. Hydrides and organometallic compounds of metals of Groups I-III of the Periodic Table are disclosed as activators or cocatalysts. Typical of the cocatalysts are tri-n-octylaluminum and dimethylethylsiloxalanes.
U.S. Pat. Nos. 4,105,842 (Aug. 8, 1978) to Nicco et al, 4,298,717 (Nov. 3, 1981) to Machon, and 4,263,170 (Apr. 21, 1981) to Bujadoux disclose the use of trialkylaluminum, halogenated dialkylaluminum, and alkylsiloxalanes as useful cocatalysts in high temperature ethylene polymerization.
British patent application No. 2,057,468A (published July 24, 1981) of Mitsubishi Petrochemical Company describes the use of siloxalanes as useful cocatalysts in high temperature ethylene polymerization and copolymerization.
As far as the inventors are aware, the only prior cocatalysts used in combination with transition metal catalysts in high temperature polymerization of ethylene are aluminum compounds with the formulas R.sub.3 Al, R.sub.2 AlCl, and R'.sub.3 SiOAlR.sub.2, where R and R' are alkyl groups.